Conventionally, inorganic photo-conductive materials such as Se, CdS, ZnO and the like have been widely used in the photosensitive layer used in a photo-receptor for electrophotography. On the other hand, the researches for superseding these inorganic photo-conductive materials with organic photo-conductive materials have been widely carried out these years. Compared to the inorganic photo-conductive materials, the organic photo-conductive materials have advantages that they can provide a photosensitive layer being much flexible and easy to manufacture and having stable electrophotographic properties, at the lower cost. Accordingly, many proposals for the organic photo-conductive materials have been offered recently. Among them, a photo-receptor whose photosensitive layer comprises separately provided carrier generating layer and carrier transporting layer has been put to practical use, since it is highly sensitive and diverse options are available to select starting materials for it, in accordance with the specific functions.
The separated function type photo-receptor usually comprises a carrier generating layer and a carrier transporting layer which are disposed, in this sequence, on an electroconductive support by coating, so that the most of incident light can be absorbed in the carrier generating layer to generate carrier and the generated carrier can be injected into the carrier transporting layer without deactivation due to recombination and capture.
For the purposes, including the prevention of deterioration of electrical properties in the photosensitive layer, and the prevention of injection of charge from the electro-conductive support into the light-sensitive layer, the provision of the subbing layer bewteen the electro-conductive support and the photosensitive layer has been being studied.
The known resins used in the subbing layer include nitrocellulose resins (cf. Japanese Patent Publication Open to Public Inspection, hereinafter, Japanese Patent O.P.I. Publication, No. 3544/1973), polyvinylbutyral resins (cf. Japanese Patent O.P.I. Publication Nos. 28837/1975 and 36258/1984), polyvinyl alcohol resins (cf. Japanese Patent O.P.I. Publication No. 100240/1977), nylon resins soluble in alcohol solvent (cf. Japanese Patent O.P.I. Publication No. 25638/1977), polyvinylformal resins (cf. Japanese Patent O.P.I. Publication No. 90639/1982), water-soluble polyvinylbutyral resins (cf. Japanese Patent O.P.I. Publication No. 106549/1983), blends of polyvinylbutyral resin and phenol resin (cf. Japanese Patent O.P.I. Publication No. 36259/1984), and others. Also, the following resins are also known: polyvinylmethylether, polyvinylimidazole, polyethyleneoxide, polyurethane, polyamide, melamine, polyvinylpyrolidone, cellulose ester, casein, gelatin and othre resins.
However, the provision of subbing layer usually deteriorates the photo-conductivity in the photosensitive layer, inevitably jeopardizing the sensitivity of photo-receptor for electrophotography.
As the higher sensitivity of photo-receptor has been realized as a result of recent study of separated function type photo-receptor, the exertive effects of the properties of a subbing layer as well as the thickness variation of the similar layer on the properties of photo-receptor have been increasingly greater. Accordingly, there is a mounting requirement for the development of the subbing layer which without deterioration of sensitivity can improve the properties including the dark attenuation properties, charge acceptability, durability and the like as well as of the subbing layer featuring the wider range of allowable layer thickness.